A CPW fed quad-port MIMO DRA for sub-6 GHz 5G applications

The present work investigates a novel four-port, multiple-input multiple-output (MIMO), single element dielectric resonator antenna (DRA) for sub-6 GHz band. The DRA is designed and fabricated into a symmetric cross shape and fed using a coplanar waveguide (CPW) feed. A single radiator with four ports is rarely found in the literature. The -10 dB impedance bandwidth covered by the antenna is from 5.52 GHz to 6.2 GHz (11.6%) which covers fifth generation (5G) new radio (NR) bands N47 and wireless local area network (WLAN) IEEE 802.11a band. The isolation between orthogonal ports is about 15 dB while the isolation between opposite ports is 12 dB. The radiation pattern of the proposed antenna is bidirectional due to the absence of a ground plane below the DRA. The orthogonal modes excited in the DRA are TEδ21x and TE2δ1y through the four symmetrical CPW feeds. The simulated and measured results of the proposed design show that MIMO characteristics are achieved by pattern diversity between the ports. Due to the perfect symmetry of the design, the proposed work could be extended to MIMO array applications as well.


Introduction
The present fourth generation (4G) mobile standard requires the demand for a high data rates and link reliability [1]. Such features can be achieved using multiple transmit and receive antennas at both ends without using extra spectrum and power [1,2]. However, the present long-term evolution (LTE) and 4G no longer keep up with the ever-increasing demand for low latency and high spectral efficiency. The fifth-generation new radio (5G NR) can deliver 100 times faster data rates as compared to 4G-LTE with a latency of less than 1 ms [3]. Many research projects are also initiated towards the sixth generation (6G) [4][5][6]. At present, to meet the existing 5G demand new design approaches and novel concepts for antenna design are necessary. Some research works have recently been performed on designing antenna arrays for 5G networks using traditional antenna structures [7][8][9]. However, due to the size constraint of today's wireless devices, it is challenging to design multiple-input multiple-output (MIMO) antennas with small size and low mutual coupling [8][9][10]. In recent years Dielectric Resonator Antenna (DRA) has gained predominant importance in MIMO systems because of its small size, high gain, radiation efficiency, ease of excitation and fabrication [11,12]. Moreover, multiple modes can be excited in a single radiating element of the DRA. These features make it a suitable candidate for MIMO operation. A dielectric resonator can excite multiple modes in a single DR element. Each mode can carry an individual data stream at each band. For MIMO operation two modes must be excited at the same frequency band, called degenerate mode. If DRA is excited at multiple degenerated modes, distinct data streams can be transmitted simultaneously from each mode at different frequency bands [13]. However, it is challenging to excite multiple modes in a single radiating element for MIMO operation due to mutual coupling between modes. DRA elements can be excited with multiple feeding ports [14]. A six-port six-element MIMO DRA was proposed in [15], however, such an arrangement makes the MIMO antenna system occupy a larger footprint. The authors in [16] proposed an omnidirectional cylindrical dielectric resonator antenna with dual polarization covering a single band, whereas a similar dual-port, aperture coupled using a single DRA was proposed for worldwide interoperability for microwave access (WiMAX) applications in [17]. A dual-band, eight-port, eight-element MIMO DRA for wireless local area network (WLAN) application was proposed in [18] which covers two bands from 2.38-2.5 GHz and 5.7-6 GHz. The volume of the proposed design is large and bulky because the proposed design has eight resonating elements. A triple port, single element MIMO DRA was proposed for X band application [19] and an L-shape, dual-port, dual-band MIMO DRA was proposed in [20] which covered both WiMAX and WLAN bands. A two-element DRA placed back-to-back with four ports was proposed in [21]. The proposed design is used for a sub-6 GHz band having a bi-directional diversity pattern. All these works have good addition to the literature, but they can be extended for multiple ports to achieve enhanced MIMO performance. In this article, we propose a novel plus shape, four-port, single element, MIMO DRA which covers the WLAN band from 5.52 GHz to 6.2 GHz. To reduce the mutual coupling between all four ports, four symmetrical coplanar waveguides (CPW) feeds are used to excite the orthogonal modes (TE x d21 and TE y 2d1 ). The advantage of the proposed design is its simplicity, and scalability and can be used for MIMO array applications as well. To the extent of our knowledge, it is the first design of a four-port MIMO DRA with a single resonator having bi-directional pattern diversity for sub 6 GHz 5G band. A single radiator with four ports is rarely found in the literature due to high mutual coupling between ports with a single radiating element which is challenging to be tackled. To resolve this issue, the proposed DRA is fed by four ports. For orthogonal ports (Port 1 and Port 2) the excitation of orthogonal modes (TE x and TE y ) resulted in low mutual coupling. For non-orthogonal ports (port 1 and port 4) the modes excited are the same but DRA is not a rectangular DRA but is cut into a 'plus' shape. By doing so, the portion of the DRA where the internal fields were interfering gets cancelled. In the 'plus' shaped DRA the fields excited by the non-orthogonal ports are not interfering with each other and hence low coupling has also been achieved between the non-orthogonal ports.

Antenna design
The proposed design is composed of a plus-shaped DRA of the dimension of 28×5 mm 2 and a height of 10mm as shown in Fig 1. The DR material is Eccostock1 Hik500f having a permittivity of 10 and loss tangent of 0.002. The DR is placed on an FR-4 substrate having a permittivity of 4.4 and dimensions of 55×55×1.6 mm 3 (2.11λ g × 2.11 λ g × 0.061λ g , where λ g is the guided wavelength at 5.52 GHz). A partial ground plane of dimension 55×10 mm 2 with some modifications for isolation improvement is etched on the top of the substrate. CPW feed lines are etched on top of the substrate. The feed lines have two-step impedance matching, the first step includes a CPW feed line with a width of 3 mm and a length of 6.5 mm having a gap c = 2.5 mm between the ground and CPW for 50 O impedance matching. The second step is a feed line of length 14.6 mm and width of 1 mm to excite the DR. In previously published works, the DRA is usually placed on a ground plane which results in a unidirectional radiation pattern. In [17] the authors have placed two DRAs back-to-back to achieve a bi-directional radiation pattern. However, in the proposed work the lack of ground plane below the DRA; a bidirectional radiation pattern is easily achieved.
By using the dielectric waveguide model (DWM) [22], the resonance frequency of a rectangular DR for the TE x d21 for the said dimensions was calculated to be 5.5 GHz. To make the DRA a four-port MIMO with acceptable bandwidth and isolation, the rectangular DR was converted to a plus-shaped DRA. This changed the resonance frequency to 5.8 GHz. The excited modes inside the plus-shaped DRA are shown in Fig 2. The figures clearly show the excitation of TE x d21 and TE y 2d1 modes by all four ports.

Parametric study
The performance of the proposed MIMO antenna depends on various parameters. However, in this research, only those parameters are considered that significantly affect the proposed MIMO antenna performance. Additionally, due to the symmetry of the design, results of reflection coefficient S 11 , while S 21 (orthogonal ports) and S 41 (non-orthogonal ports) are shown for isolation. Effect of DR size on isolation and impedance matching. The isolation, impedance matching, and resonance frequency of the antenna are dependent on the DR shape and size. Fig 3(A) shows the effect of the DR segment on the performance of the antenna. This figure indicates that without DR, there is no resonance at 5.8 GHz. As the CPW feed line gives resonance at 2.4 GHz and 4 GHz, isolation at these bands is poor as clear from the figure. After placing a square shape DR having dimensions of 28×28×10 mm 3 on the substrate, the Sparameters reveal that the antenna is resonant at 5.5 GHz, 6.4 GHz and not at the band of interest, as shown in Fig 3(A). Also, the isolation between non-orthogonal ports (S 41 long

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dashed lines in black) is very poor. Once the square shape DR is replaced with plus shape DR, the resonance frequency is adjusted to 5.8 GHz and the isolation between non-orthogonal ports (S 41 long dashed lines in blue) improves considerably, as given in Fig 3(B). In both cases, isolation between orthogonal ports (S 21 short dashed lines in black and blue) is about 15 dB.
Effect of feedline length 'e' on impedance matching and isolation. The feed line length 'e' as shown in Fig 1(B) plays an important role in deciding the resonance frequency and isolation between the ports and is shown in Fig 4(A) and 4(B). Both figures show that by increasing 'e' the resonance frequency decreases while the isolation between the orthogonal ports (S 21 dotted lines) and non-orthogonal ports (S 41 solid lines) decreases. The optimum value of 'e' is chosen to be 14.6 mm at which the resonance frequency is 5.8 GHz while S 21 and S 41 are 16.01 dB and 11.93 dB, respectively.
Effect of tilting angle 'θ' on the impedance matching and isolation. The effect of the tilt angle θ (shown in Fig 1(A)) which is the angle between the DR and the x-axis is shown in  , the tilting angle slightly shifts the resonance frequency towards the right while its effect on the mutual coupling between the orthogonal ports (S 21 ) and non-orthogonal ports (S 41 ) is insignificant (Fig 5(B) and 5(C)).
Effect of CPW gap 'g' on the impedance matching and isolation. The gap 'g' between the feedline and the ground plane (Fig 1(A)) plays a significant role in the impedance matching and mutual coupling between the ports as shown in Fig 6. Fig 6(A) reveals that by increasing the gap 'g' matching improves. The coupling between the non-orthogonal ports (S 41 ) also improves by increasing 'g' as shown in Fig 6(B), while coupling between orthogonal ports (S 21 ) remains almost the same as shown in Fig 6(C). A value of g = 3mm has been selected for the proposed design. The simulated and measured S-parameters of the proposed MIMO antenna are shown in Fig 8. Fig 8(A) shows the simulated and measured reflection coefficient of the antenna. As clear from the figure, there is a mismatch in simulated and measured results due to fabrication

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tolerance. However, the matching is still better in the frequency band of interest, i.e., 5.8 GHz .  Fig 8(B) shows isolation between different ports. It is seen in the figure that the isolation between orthogonal ports P1-P2, P1-P3, P2-P4 and P3-P4 is well below 15 dB in the band of interest while the isolation between non-orthogonal ports P1-P4 and P2-P3 is up to 12 dB in the band of interest.
The isolation between the ports can be explained with the help of surface current flow on the ground plane and E-field distribution inside the DR. As evident from Fig 9(A), when port 1 is excited and other ports are terminated with 50 O, most of the current is confined to the exciting port. With the optimized shape of the ground plane, the current flow towards the rest of the ports is minimal. The same is the case when port 2 is excited as shown in Fig 9(B). However, the cause of coupling between ports is highly dependent upon the excited modes inside the DR.
The modes excited are orthogonal (TE x d21 and TE y 2d1 ) when orthogonal ports (P1-P2, P1-P3, P2-P4, P3-P4) are active resulting in low coupling between these ports as shown in Fig 10(A). From this figure, it can be deduced that the fields excited are the orthogonal fields (TE x d21 and TE y 2d1 ) resulting in low coupling between these ports. On the other hand, when non-orthogonal ports (P1-P4, P2-P3) are active, they result in the excitation of non-orthogonal modes TE x d21 or TE y 2d1 resulting in high coupling between these ports because of the same polarization as shown in Fig 8(B).
The simulated 3D radiation pattern of the proposed four-port MIMO DRA is shown in Fig  11. The pattern diversity is significantly visible which is imperative for MIMO operation. The

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The correlation coefficient (ρ) is a measure of how much communication channels are correlated or isolated from each other. This performance metric takes into account how much the radiation patterns are correlated when operated simultaneously. The envelope correlation coefficient (ECC) is the square of ρ, and can be calculated using two methods (Eqs (1) and (2) of [3]), scattering parameters and 3D radiation pattern [23].
In the first method ECC only depends upon surface currents. This method is simple us use but do not account for coupling caused by radiation pattern. In the second method, ECC also accounts for coupling caused by 3D patterns. This method is more accurate however, it is difficult to use because it requires 3D pattern measurement [24].
φÞ � E � 2 ðy; φÞdO ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi where � E i ðy; φÞ is the three-dimensional field radiation pattern of the antenna when the ith port is excited, and O is the solid angle. ( � ) is the Hermitian product operator.
The results of ECC by using Eq (2) are shown in Fig 13. It can be observed that in the band of interest ECC for orthogonal (P1-P2) is below 0.02 while for non-orthogonal ports (P1-P4) the ECC is below 0.05. The ECC value obtained by using Eq (1) is 0.042 for orthogonal ports (P1-P2) and 0.06 for non-orthogonal ports (P1-P4). The above values clearly show that the ECC for the orthogonal ports (P1-P2) is better than the non-orthogonal ports (P1-P4) by using both Eq (1) and Eq (2), as expected.
The diversity gain (DG) specifies the quality and reliability of a wireless communication link [25]. A high value of DG close to 10 dB is desirable in the operating band. The DG is calculated from the ECC by using Eq (3). The simulated and measured DG of the proposed antenna is shown in Fig 14. It is clear from the figure that the DG is close to 10 dB in the band of interest. . The MEG is calculated from the S-parameters by using Eqs (4) and (5). For the proposed antenna the simulated and measured MEG is presented in Fig 15. As clear from the figure the ratio of the MEGs is below 3 dB resulting in acceptable diversity performance.
The channel capacity loss (CCL) approximates the transmission of the maximum limit of the message signal without communication channel loss. The acceptable value of CCL should

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A comparison of the proposed work with previous work is given in Table 1. The proposed MIMO antenna compared to other MIMO antennas available has a single DR element with four ports thus considerably decreasing the size of the MIMO system with reasonable antenna and MIMO performance.

Conclusion
This paper presents the design, implementation, and measurement of a CPW fed, quad-port MIMO DRA with a single resonator for 5G N47 and WLAN applications. The proposed design is a symmetrical plus-shaped DR and excited through four independent CPW feed lines

Author Contributions
Formal analysis: Jamal Nasir.